How to get a high mark in your french letter.
1)know your tenses.know the passe compose, present, both future tenses and the conditional tense. there are verbs that are more common, but i don't have a list of them at the moment
2)Use idioms. Knowing one or two idioms that can work in a few situations will help with the flow of the letter.
3)Use negative sentences.
4)Use descriptive words. when describing something try to say stuff like 'the leather dress' or 'the red bicycle' rather than just the words on their own.
5) Use reflexive verbs. If you know how to use reflexive verbs, they will get you more marks. Examiners will be particularly impressed if you can use them in the passe compose.e.g I enjoyed myself= Je me suis bien amusée.
6) Try to use the conditional tense. Sentences such as 'I would go out with my friends, if I wasn't sick.' The vocabulary is simple enough, but the examiner will be impressed that you know how to use this tense.
7) Instead of saying you did something, say how it made you feel. For example; We went on a school exchange. It was fun. I really enjoyed myself. This bulks out your letter as well as showing off your vocabulary.
8) Justify what you say. e.g Don't just say you like/don't like a school subject. Instead of saying' I don't like maths' and then moving on, say why you don't like it. 'Maths is my least favourite subject because...'.
I'm sure there are plenty of other things that can get you marks, but these are the main ones that I know of. Hope it helps! :)

Really the only way to get better at comprehensions is to practice. If there is a word you don't understand, you can usually figure it out by context. There are some apps that help with vocabulary such as duolingo.